A sandwich panel is a panel comprising two generally parallel walls, which are held fixed in relation to one another by means of a core. The walls are relatively thin and may have a thickness of less than 1 millimeter. The walls are for example made of aluminium or carbon. The core of the sandwich panel may be an insulating foam or a honeycomb structure. Because of this structure, a sandwich panel cannot have tapped holes. This is because it is impossible to machine an internal thread either in walls that are this thin or in a foam or honeycomb structure. However, it is still necessary to have screwed joints in order to attach various elements to sandwich panels. One solution involves using rivet nuts, also called anchor nuts or thread inserts. A rivet nut comprises a cylindrical body, also called the stem, and a head, also called the collar, which extends in the radial direction at one end of the body. The head forms an annular contact surface for contact with the wall of the sandwich panel. A bore extends axially inside the body from the head. The bore can be blind or open-ended. The body comprises a portion in which the bore has a tapped thread and a portion which is deformable.
The procedure for installing a rivet nut in a wall of a sandwich panel is as follows. In a first step, a blind hole is machined in the wall of the panel. The diameter of the hole is substantially equal to the outer diameter of the body. In a second step, the rivet nut is screwed onto the threaded shank of a rivet nut setting tool and positioned in the hole in the wall, the collar coming to press against the wall of the panel. The nut is then set, the rivet nut setting tool exerting a tensile force on the tapped portion of the rivet nut in the direction of the collar. The rivet nut setting tool can then be unscrewed from the rivet nut. After setting, the deformable portion of the body of the nut forms a flange which comes to press against the internal surface of the wall of the sandwich panel. The rivet nut is thus attached to the wall in that it squeezes the wall between the flange and the collar of the rivet nut. A rivet nut is described as blind if it is attached from just one side of the wall.
One problem with rivet nuts is that their collar protrudes from the surface of the panel wall. This protrusion prevents the surface of the panel wall from being brought into contact with an element to be attached thereto. Direct contact of this type may be desirable in order to distribute the transmission of forces between the panel and the element attached thereto, or in order to increase thermal exchanges between the parts. Rivet nuts which reduce the thickness of the protrusion, specifically countersunk rivet nuts and rivet nuts having low-profile heads, are available. These two types of nut have a countersunk head which becomes embedded in the wall of the panel by local deformation during setting. Nonetheless, even with rivet nuts having low-profile heads, also called slender head rivet nuts, it is impossible to completely negate the protrusion. However, in certain fields, such as astronautics, flush tolerances can be of the order of a tenth of a millimeter. Furthermore, given that the annular width of the collar of countersunk nuts and nuts having low-profile heads is smaller than that of flat-headed nuts, the resistance of these nuts to being pushed inside the panel is reduced.
It is possible to reinforce this push-through resistance by injecting adhesive into the panel at the location of the rivet nut. Where a honeycomb sandwich panel is concerned, the quantity of adhesive to be injected in order to completely embed the rivet nut is difficult to estimate. In particular, creating the blind hole can have opened a plurality of cells of the honeycomb. One solution is to create two additional holes near the rivet nut. A first hole is used for injecting adhesive, and a second hole acts as a vent. Adhesive is injected through the first hole until it starts to emerge from the other. This method presents the notable drawback that additional holes must be machined. This therefore increases the time required for installing the rivet nut. Moreover, the holes weaken the panel and can present leaktightness problems. One other drawback is that the adhesive which has overflowed from the second hole must be cleaned off the panel.